1. Field of Invention
The invention relates to an automatic transmission mounting on a vehicle. More specifically, it relates to the arrangement of the each transmission structural element in the gear train.
2. Description of Related Art
An automatic transmission comprises a planetary gear set, and frictional elements, which are clutches and brakes, for changing a transmission route of a power through the planetary gear set. Conventionally, the automatic transmission is required to multiply the gear ratios for an improvement in driving performance and fuel consumption. Recently, an automatic transmission, which establishes gear ratios of five forward speeds and one reverse speed, has been developed. To avoid complications in control, in which more than three engagement elements are needed to simultaneously engage/disengage (that is clutch to clutch operation) in a shift up/down between certain gear ratios, based on an increase in the number of engagement elements, and enlargement of the gear train, U.S. Pat. No. 5,342,257 describes a gear train in which one of three sets of planetary gears is a double pinion type, five speeds are achieved with three clutches and two brakes, and a clutch to clutch operation between more than three frictional elements is not needed.
In the aforementioned conventional transmission, a decelerated rotation decelerated from an input rotation, which is achieved from a double planetary gear, is inputted to a certain element of a planetary gear through a clutch so that the multistage is established with few engagement elements. But, in this transmission, there is a problem. To improve fuel consumption and driving performance, it is necessary to broaden a gear ratio breadth in the entire transmission mechanism in addition to the multiplying. However, in the conventional transmission, it is difficult to achieve a greatly decelerated rotation, that is, it is difficult to broaden the breadth between the gear ratios because the first speed is established by inputting the input rotation through a clutch to a predetermined element and fixing another element by a brake. A transmission solving the aforementioned problem, a gear train, which achieves six forward speeds and one reverse speed with three clutches and two bakes, is suggested in U.S. Pat. No. 5,106,352. In the gear train, six speeds are achieved by a rotation decelerated from the input rotation is inputted to two shift elements of a ravegneaux type planetary gear set and a non-decelerated rotation is inputted to another shift element. In the conventional transmission, a first speed established by the decelerated rotation is inputted through a clutch to a predetermined element and another element is fixed by a brake. Therefore, a relatively large decelerated rotation is achieved. As a result, a multiplied automatic shift having a broaden gear ratio breadth is achieved.
In the gear train in the just discussed conventional transmission, the gear ratio breadth is broad and the six speeds have proper gear ratio steps. However, two power transmission routes, which need a high strength for transmitting a high torque increased by a deceleration at the reduction planetary gear, are needed. Therefore; the transmission routes are needed to be compactly structured to avoid an increase in the size and weight of the transmission. To address this point, in the conventional transmission, the reduction planetary gear and a clutch for transitting a non-reduction torque are arranged between two clutches for transmitting reduction torque and a planetary gear set. Therefore, there is no particular consideration according to the high torque transmission.